An example structure of a level shift circuit of the related art is shown in FIG. 9. The level shift circuit in this example level-shifts (level-converts) an input signal IN with a low voltage amplitude of, for example, 3.3 V to a signal with a high voltage amplitude of, for example, 6.5 V. As is apparent from FIG. 9, the level shift circuit includes two bias shift units 101 and 102, a level shift unit 103, and an output unit 104.
If the level shift circuit of this type is formed of transistors with large characteristic variations, for example, TFTs (Thin Film Transistors), in addition to the input signal IN, a reverse-phase signal XIN of the input signal IN must be input in order to reliably activate the circuit. Alternately, in place of the reverse-phase signal XIN, a reference potential Ref having an intermediate level of the amplitude of the input signal IN must be input. The input signals IN and XIN (or Ref) must be input directly to a source of the input stage of the level shift unit 103.
As described above, in the structure in which, in addition to the input signal IN, the reverse-phase signal XIN is externally input, an external IC is required for generating the reverse-phase signal XIN based on the input signal IN, and the number of input signal terminals required for the level shift circuit is two. On the other hand, in the structure in which the reference potential Ref is input, an external Ref power supply is required.
If the externally input signals IN and XIN (or Ref) are input directly to the source of the input stage of the level shift unit 103, a current flows in an external input terminal, thus causing overcurrent into the external IC when the power supply is turned on or the like. This may induce latchup on the external IC.
The present invention has been made in view of the foregoing problems, and it is an object of the present invention to provide a level shift circuit which can reliably level-shift a single-phase input signal when it is formed of transistors with large characteristic variations, a display apparatus using the level shift circuit, and a portable terminal including the display apparatus as a screen display unit.